Tunnel Love

It is 20 years since the official opening of the Channel Tunnel. At over 31 miles long, it is hailed as one of the greatest feats of civil engineering ever.

But how does the ‘chunnel’ compare to Glasgow’s own tunnels?

The Glasgow Subway is the third oldest underground railway in the world (after London and Budapest). So you’d expect the tunnelling process of 1896 to be very different to today’s. But no…

“In essence it was much the same,” says Neil Gatenby, SPT’s project engineer who is part of a team overseeing infrastructure on the Subway’s £288 million renovation.

“Tunnels are created using a shield, which protects the excavation, with the support structure dropped in behind. I don’t know which engineer came up with the concept, but it is still used in tunnelling round the world.”

The Chunnel might have been a far larger scale of project, with more modern equipment, but it is remarkable to think that the two Subway tunnels under the Clyde were created in much the same way.

However, there are oddities to the Glasgow system. It’s unusually narrow with tunnels of a diameter of just 3m (many London tunnels are twice the size) and the railway is just 4ft wide.

“London has soft clay throughout, it’s perfect for tunnelling,” says Neil. “In Glasgow, north of the river you have hard rock, but south of the river the geology is different – it’s loose soil.

The solution the Victorian engineers came up with was a construction system known by “cut n cover” which simply involved the following sequence:

driving timber piles from street level

excavating down to crown level

pouring concrete to form the crown

backfilling to ground level

This allowed early re-opening of the road and minimised disruption. Thereafter the tunnels were excavated beneath the concrete crown with the brick sidewalls and concrete invert formed as they progressed.

“It’s the way most city subways have been created round the world,” says Neil. “But it can be incredibly disruptive. The tunnels follow the roads so you have to close the roads while you create the cover. It’s one reason why the system here hasn’t been expanded.”

In some instances the tunnelling works were undertaken under compressed air due to the nature of the soils and the high water table.

It was of course a dangerous business, especially where water was involved. “While tunnelling under the Clyde they had problems with the air pressure, and when they got it wrong there were blow outs, which brought the water in,” Neil explains.

Lessons were clearly learned however. The Channel Tunnel project wasn’t without fatalities – two men lost their lives during the construction. But its safety record, considering the scale and complexity of what was involved, is remarkable.

And that in part comes down to the work of engineers in earlier tunnelling projects like Glasgow’s.